This invention relates to optical fibre communication systems and, in particular to communication systems which employ solitons or soliton-like pulses for data transmission. It is also applicable to systems in which the launch pulse may be phase modulated return-to-zero (RTZ). In such systems, which are not obviously soliton-like, after travelling a distance, the pulses are transformed into soliton-like pulses.
It has recently been shown that a new class of optical solitons occur in dispersion managed systems where alternating sections of negative (anomalous) and positive (normal) dispersion fibre are used. (See, for example, Suzuki, M., Morita, I., Edagawa, N., Yamamoto, S., Taga, H., and Akiba, S., xe2x80x98Reduction of Gordon-Haus timing jitter by periodic dispersion compensation in soliton transmissionxe2x80x99, Electron. Lett., 1995, 31, (23), pp. 2027-2029, Smith, N. J., Knox, F. M., Doran, N. J., Blow, K. J., and Bennion, I., xe2x80x98Enhanced power solitons in optical fibres with periodic dispersion managementxe2x80x99, Electron. Lett., 1996, 32, (1), pp54-55 and Smith, N. J., Forysiak, W., and Doran, N. J., xe2x80x98Reduced Gordon-Haus jitter due to enhanced power solitons in strongly dispersion managed systemsxe2x80x99, Electron. Lett., 1996, 32, (22), pp2085-2086.
In a further paper entitled xe2x80x98Energy scaling characteristics of solitons in strongly dispersion-managed fibresxe2x80x99, Opt. Lett., 1996. 21, (24), ppl981-1983, Smith et al. derived an empirical relationship for the enhanced power of these solitons, where the average dispersion is anomalous and significantly less (in magnitude) than the dispersion in the two segments. These lossless calculations showed the importance of the launch point in the map (the minimum chirp is at the centre of either section), but did not establish the exact pulse shape, nor the long term stability of the pulses.
We have discovered that by using dispersion management, in which an optical communication system uses alternative sections of fibre of opposite sign of dispersion, that transmitted pulses are not distorted (neither dispersively nor effectively nonlinearly) provided the correct form of the pulse is selected. It is possible to have stable pulses (solitons) where the net dispersion is zero, normal or anomalous. There are no solitons for normal dispersion, but pulses are also stable in this regime. This permits wavelength multiplexing around the zero dispersion since, although the dispersion depends on wavelength, it is unavoidable that both signs will occur. However, the new arrangement permits solitons to be used for a wide range of wavelengths.
We have found that the shape of pulse is significant. For these systems it is important to pre-chirp the pulse in an appropriate way. The degree of chirp and pulse duration depends on the data rate required and how the map is designed.
We have also discovered that for zero net dispersion there appears a preferred pulse duration for a particular map. The ratio                               β          ¨                ⁢                  xe2x80x83                ⁢        l                    τ        2              ∼    4    ,
where {umlaut over (xcex2)} is the fibre dispersion, xcfx84 is the pulse duration and l is the fibre length. It means that the system (for the zero dispersion case) is specified by the pulse duration (effectively the data rate) and the dispersion of the fibres i.e. the length of each section can be immediately inferred. For example if xcfx84=20 ps (10 OGb/s) and {umlaut over (xcex2)}xcx9c20 ps2/km (standard fibre), then the fibre lengths should be 80 km. Alternatively, if {umlaut over (xcex2)}=1 ps2/km (dispersion shifted fibre typical number) then 1600 km is ideal. Numerical modelling indicates that there are stable nonlinear transmission pulses for periodically dispersion managed systems where the path average dispersion may be either anomalous, zero, or even normal.
A new class of stable pulses is demonstrated to exist when the average dispersion is zero or even normal. The discovery of these stable pulses allows the use of solitons in WDM systems around the zero (average) dispersion, where due to dispersion slope effects both signs of dispersion are inevitable.
According to one aspect of the present invention there is provided a soliton or soliton-like pulse-based optical communication system comprising a length of optical fibre divided into a plurality of sections wherein the average dispersion of the length of fibre is significantly different from the dispersion of each section. According to a further aspect of the present invention there is provided a soliton or soliton-like pulse-based optical communication system comprising a length of optical fibre divided into a plurality of sections wherein the average dispersion of the length of fibre is significantly different from the dispersion of each section and wherein the pulse duration t is substantially equal to xc2xc{umlaut over (xcex2)}l where {umlaut over (xcex2)}is the fibre dispersion, xcfx84 is the pulse duration and l is the fibre length.